The present invention relates to a fluidized-bed bioreactor, particularly to a bioreactor for separating contaminants from liquids and degrading the separated contaminants.
Conventional fluidized-bed bioreactors suffer from operational drawbacks in that the media or carriers of the fluidized bed may be subject to excessive buildup of biologically active materials (or xe2x80x9cbiomassxe2x80x9d) and precipitates, thereby causing compromised flow distribution, excessive media and/or biomass carryover, crusting, increased clogging and the like. Specifically, if not properly limited, biomass and precipitate buildup causes uncontrolled bed expansion in the bioreactor, which is detrimental to system performance. Uncontrolled media bed expansion in a fluidized bed biological reactor can also result in an undesirable loss of media.
Media bed expansion can, under certain circumstances, be limited by the application of shear toward the top of the fluidized bed, but the success of such a control strategy depends upon whether excess biomass and suspended solids can be transported to the top of the fluidized bed. More specifically, it is recognized that such transportation of excess biomass and suspended solids toward the top of the bed is promoted by several dominant mechanisms. For example, media grains that are coated with thicker layers of biomass tend to have an overall particle density that is less than the average particle density within the fluidized bed. Those particles, therefore, are transported to the top of the fluidized bed by virtue of upward moving fluid flow as well as the reduced particle density. Also, the movement of media particles within the fluidized bed tends to cause shear to help dislodge biomass from the surface of the media particles. That sheared biomass is then transported in an upward direction as it is carried by the bulk fluid flow.
Nevertheless, several conditions tend to limit the ability to apply shear at the top of a fluidized bed. For example, shear-resistant biological growth can limit the effect of shear at the top of the fluidized bed reactor, especially when conditions within the fluidized bed reactor result in a rapid growth of filamentous organisms or xe2x80x9cbloomsxe2x80x9d. Under such conditions, the transport of media particles with thick biomass coatings is inhibited, and excessive biomass accumulation can occur in the lower and central regions of the fluidized bed. Excessive bed growth can therefore result, as the shear applied at the top of the fluidized bed does not impact the lower and central regions of the bed adequately.
Also, the impact of shear at the top of the fluidized bed reactor can be compromised if a shear-resistant or high-density precipitate (e.g., an inorganic film) forms within the fluidized bed. Such precipitate can accumulate throughout the fluidized bed, thereby encouraging uncontrolled bed growth. For example, iron hydroxide may be precipitated if ferrous iron is oxidized under aerobic conditions.
Attempts have been made in the past to overcome this long-standing problem of uncontrolled bed growth. The operation of the fluidized bed reactor can be interrupted periodically in order to conduct chemical treatment of the media bed. For example, acid washing of the media can be conducted in order to remove iron precipitate, and shock treatment can be conducted with sodium hypochlorite. The operation of the fluidized reactor can also be interrupted periodically to transfer the media to a holding tank for later return to the fluidized bed reactor. Shear is thereby imparted in each of these transfer operations so as to loosen attached solids from the media. Such loosening of the solids from the media by media transfer can be enhanced by external washing or chemical treatment. Finally, the replacement rates of the media can be elevated in order to replenish the media that tends to be lost as a result of uncontrolled bed expansion.
U.S. Pat. Nos. 4,892,818 and 4,904,600, both to Floyd Ramp, describe a fluidized bed bioreactor with recirculating wash liquid. Wash liquid is forced from the bioreactor to a separator for contaminant removal. The wash liquid is recycled into the bioreactor by a pump. A retaining screen provided at the wash liquid outlet prevents circulation of the packing material.
U.S. Pat. No. 4,545,909, issued to Bernard Atkinson et al., describes a bioreactor for treating sewage. Media and the attendant water and biomass is delivered to a straining device, separated from the water, and transported to a machine that separates biomass. The separation machine separates biomass by compression, intense vibration, or other mechanical methods. Alternatively, a chemical or biological separation method, such as extended aeration, is used.
It has been found that application of conventional techniques to remove excess biomass from the slurry of biomass, water and media are sometimes insufficient to help (1) overcome bed expansion caused by shear resistant biological growth and (2) reduce the formation of precipitates formed by the microorganisms found in oxygen deficient zones of bioreactors. Thus, there remains a need in the industry for an improved system for separating accumulated biomass from a slurry of a fluidized-bed bioreactor to inhibit uncontrolled bed expansion and precipitate accumulation.
It is therefore an object of the invention to provide a system for controlling bed expansion. Other objects and advantages of the invention will become apparent to those skilled in the art from the drawings, the detailed description of preferred embodiments, and the appended claims.
One aspect of the invention provides a reaction chamber adapted to contain a fluidized bed having a slurry of liquid, media and biomass. A lift and motive fluid are provided to urge slurry from the reaction chamber through a passage from a slurry inlet to a slurry discharge. The height of the slurry inlet is preferably adjustable with respect to the bottom of the reaction chamber.
In another embodiment, the lift urges the slurry from the bioreactor through a passage from the slurry inlet to a biomass separator. The excess biomass is preferably removed via a biomass discharge and the slurry, with a reduced concentration of biomass, is preferably returned to the bioreactor.
In yet another embodiment, a portion of the slurry that exits the biomass separator is preferably used as part or all of the motive fluid that urges additional slurry through the passage from the slurry inlet to the biomass separator.
In operation, preferably by adjustably positioning the slurry inlet at a height above the bottom of the reaction chamber, a portion of the slurry in the fluidized bed is urged from the fluidized bed through the passage. Optionally, excess biomass is separated from the slurry and the slurry is then discharged through a slurry discharge and returned to the fluidized bed. The separated biomass is preferably concentrated and removed from the bioreactor via a biomass discharge.